Being friendly puts monkeys at risk in times of revolution

A friend in need…

Suzi Eszterhas/NaturePL

By Colin Barras

Being too friendly can be costly. When a new alpha male takes over, female capuchin monkeys are more likely to lose their offspring to infanticide if they have an extensive network of social contacts than if they don’t.

This new finding suggests sociable primates don’t necessarily fare better than non-sociable ones when it comes to raising offspring.

Group-living mammals have plenty to gain from being sociable, says Urs Kalbitzer at the University of Calgary in Canada. They can have better access to food and more protection from predators, as they often take up a position near the centre of the group. These advantages should help the most sociable females raise more infants to adulthood.

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But some researchers have suspected that being sociable carries a cost when the group’s alpha male loses his position to a rival. The usurper can kill offspring he hasn’t fathered so that adult females will become receptive to his sexual advances.

One idea is that the alpha male is more likely to kill the offspring of females at the group’s social core, as less sociable females on the periphery may escape his attention. To test the theory, a team led by Kalbitzer and his colleague Linda Fedigan looked at data from wild communities of white-faced capuchins (Cebus capucinus) in Santa Rosa, Costa Rica, between 2005 and 2011.

Infanticide risk

They found that an infant was, on average, more likely to survive during times of group stability if its mother was sociable. But when the alpha male’s position was threatened or if he was replaced – which happened roughly once every four years – infants born to sociable females were more at risk of infanticide.

Overall, the team found that a female’s level of sociability had no significant effect on the number of offspring she successfully raised over the course of her lifetime. In other words, the advantages of sociability seem to be counterbalanced by the price she pays during alpha male changeovers.

Surprisingly, there was no evidence that females changed how sociable they were at different times, even though becoming less sociable when the alpha male was threatened might reduce the risk of infanticide.

“Females with strong bonds may not opt out of their spatially central position in times of increased infanticide risk because peripheral positions are associated with increased predation risk,” says Oliver Schülke at the University of Göttingen, Germany. “It will be interesting to see whether, with more data accumulating, highly social females turn out to enjoy longer lives.”

There might be lessons from such studies for understanding human social networks too, says Kalbitzer. “They can reveal which features of social relationships are relatively ancient, and which selection pressures may have acted on our ancestors.”

Journal reference: PNAS, DOI: 10.1073/pnas.1608625114